In the field of refrigeration and air conditioning, 1,1,1,2-tetrafluoroethane (R134a) that is a hydrofluorocarbon (HFC) and R410A that is a mixed refrigerant of difluoromethane (R32) and pentafluoroethane (R125) in a mass ratio of 1/1 are currently widely used as refrigerants for a refrigerator, a car air conditioner, a room air conditioner, industrial refrigerating machine, and the like. Although such HFC refrigerants have an ozone depletion potential (ODP) of zero, they have a high global warming potential (GWP) of 1000 or more, and therefore, their usage is limited in accordance with what is called the F-gas regulations aiming at the global environmental protection.
A hydrocarbon refrigerant, such as isobutane (R600a) and propane (R290), practically used for a refrigerator has such a suitable physical property value as a GWP as low as 20 or lower, and hence is being examined as an alternative refrigerant although it is combustible.
Differently from an HFC refrigerant and the like, a hydrocarbon refrigerant cannot be expected to show an effect of improving lubricity by the refrigerant because it does not contain, in a hydrocarbon molecule, fluorine improving the lubricity, and in addition, the hydrocarbon refrigerant has high solubility in a refrigerating machine oil and hence lowers the viscosity of the oil, namely, the thickness of an oil film is reduced, which makes a lubrication condition severer, and accordingly, a working fluid for refrigeration and air conditioning in which a refrigerating machine oil and a refrigerant are blended is required to have higher antiwear property than conventionally required.
With respect to a base oil for a general industrial lubricating oil, Patent Literature 1 proposes a lubricating oil base oil containing a synthetic ester obtained by reacting an alcohol component containing 90% by mass or more of trimethylolpropane with a carboxylic acid component that contains a monocarboxylic fatty acid having 8 to 12 carbon atoms and adipic acid, and further contains, in a total amount of 90% by mass or more, caprylic acid and/or a monocarboxylic fatty acid having 8 to 12 carbon atoms containing 90% by mass or more of caprylic acid in total, and adipic acid.